Method for producing an article from a maraging steel

ABSTRACT

The present invention relates to a method for producing an article out of a maraging steel, wherein the article is successively subjected to a solution annealing and heat treatment, wherein the steel has the following composition in weight percent: 
     C=0.01-0.05
 
Si=0.4-0.8
 
Mn=0.1-0.5
 
Cr=12.0-13.0
 
Ni=9.5-10.5
 
Mo=0.5-1.5
 
Ti=0.5-1.5
 
Al=0.5-1.5
 
Cu=0.0-0.05
 
     Residual iron and smelting-induced impurities.

The present invention relates to a method for producing an article out of a maraging steel according to the preamble to claim 1 and to an article composed of a maraging steel.

So-called maraging steels are steels whose alloy is virtually carbon-free.

The maraging steels on the one hand, have high strength and on the other, have good toughness along with good processing and welding properties. They are used as tool steels for use at elevated temperatures, e.g. with intricately shaped die-cast or injection-molded plastic tools and for producing knives and blades for the sport of fencing.

Examples of maraging steels are the hot-working steels 1.2709 and 1.6356.

DE 603 19 197 T2 has disclosed a maraging steel, which contains at most 0.01% C, 8 to 22% nickel, 5 to 20% cobalt, 2 to 9% molybdenum, between 0 and 2% titanium, at most 1.7% aluminum, between 0 and 10 ppm magnesium, less than 10 ppm oxygen, less than 15 ppm nitrogen, and the rest iron and random impurities; this maraging steel contains nitride inclusions with a maximum length of 15 μm and oxide inclusions with a maximum length of 20 μm; the oxide inclusions include spinel-type inclusions and aluminum oxide inclusions, and in the total content of spinel-type inclusions with a length of at least 10 μm and Al₂O₃ with a length of 10 μm, the percentage of spinel-type inclusions with a length of at least 10 μm is greater than 0.33. The intent of this is to take into account the objective of significantly reducing the non-metallic inclusions.

EP 1 222 317 B1 has disclosed a high-strength, stainless machining steel; it is produced by powder metallurgy and should contain a precipitation-hardenable stainless steel alloy; this alloy contains at most 0.03% carbon, at most 1% manganese, at most 0.75% silicon, at most 0.04% phosphorus, 0.01 to 0.05% sulfur, 10 to 14% chromium, 6 to 12% nickel, at most 6% molybdenum, at most 4% copper, 0.4 to 2.5% titanium, and other minor alloy additives; the rest should be composed of iron and the usual impurities; a powder metallurgy product is to be produced from this, which should contain a fine dispersion of tiny sulfide particles whose major dimension is no greater than about 5 μm. It can also be used to produce a wire.

EP 0 607 263 B1 has disclosed a precipitation-hardenable martensitic steel, which in addition to the usual small amounts of metals added to the alloy, also contains 10 to 14% chromium, 7 to 11% nickel, 2.5 to 6% molybdenum, and 0.5 to 4% copper; in this case, it can also contain up to 9% cobalt, the rest consisting of iron and the usual impurities.

EP 2 631 432 B1 has disclosed a steam turbine rotor, a corresponding steam turbine, and a turbine power plant; the steam turbine rotor is a steam turbine low-pressure last stage long blade, which is composed of a precipitation-hardenable martensitic stainless steel, which contains less than 0.1% carbon and 9 to 14% chromium as well as 9 to 14% nickel, 0.5 to 2.5% molybdenum, and 0.5% or less silicon.

The object of the invention is to create a method for producing an article out of a maraging steel, which enables an optimal ratio of hardness to toughness.

The object is attained with the features of claim 1.

Advantageous modifications are disclosed in the claims that depend thereon.

Another object is to produce an article composed of a maraging steel, which enables achievement of an optimal ratio of hardness to toughness.

The object is attained with a subject having the features of claim 9.

Advantageous modifications are disclosed in the claims that depend thereon.

In the maraging steels according to the prior art, either a high level of hardness or a high level of toughness can be achieved. A high level of toughness is possible particularly if the aging enables achievement of a high percentage of retransformed austenite. But this percentage of retransformed austenite in turn has a negative influence on the maximum achievable hardness.

According to the invention, an alloy concept has been discovered, which with a high level of hardness, nevertheless permits the achievement of a high level of toughness.

According to the invention, an alloy concept is used, which is essentially based on nickel, aluminum, titanium, and silicon as hardening elements.

In order to bring the hardness, strength, and toughness of the alloy into harmony at a high level, the focus was placed on two points, namely on the one hand, increasing the hardness and strength values by modifying precipitation densities and types. To accomplish this, according to the invention, the content of the precipitation-promoting elements aluminum and titanium was increased.

In order to increase the toughness, the percentage of retransformed austenite was increased, which was possible to achieve by increasing the nickel content.

The invention will be explained by way of example based on the drawings. In the drawings:

FIG. 1 shows the influence of the titanium content;

FIG. 2 shows the influence of the molybdenum content;

FIG. 3 shows a time/temperature diagram of a heat treatment, which consists of a solution and washing procedure followed by an air-cooling to room temperature and an aging process.

FIG. 4 shows the phase diagram of iron/nickel in equilibrium;

FIG. 5 shows the hysteresis of conversion temperatures of martensite and austenite during heating and cooling;

FIG. 6 shows Table 1, which depicts the chemical composition of steels according to the invention;

FIG. 7 shows Table 2, which depicts another composition of a powder according to the invention;

FIG. 8 shows the hardness as a function of the aging temperature for different alloys;

FIG. 9 shows the austenite content as a function of the aging temperature for different alloys;

The following is a listing of the most important alloying elements and their influence on the microstructure and properties in maraging steels and in the invention in particular.

Nickel (Ni):

Nickel is the most important alloying element in maraging steels. Since the carbon content is low in maraging alloys, the addition of Ni to Fe results in the formation of a cubic Fe—Ni martensite. Controlling the Ni content is also important because Ni is an austenite-stabilizing element and Ni is thus decisive for the formation of retransformed austenite. Ni forms intermetallic precipitations with numerous elements such as Al, Ti, and Mn and therefore plays an additional decisive role as a precipitation-promoting element.

Aluminum (Al):

Aluminum is added to maraging steels as a precipitation element. It increases the solid solution strengthening and, particularly with Ni, forms intermetallic precipitations. A higher Al content can lead to the presence of δ ferrite in the microstructure, which has a negative impact on the mechanical properties and on the corrosion resistance.

Titanium (Ti):

Titanium appears to be one of the most active elements in maraging steels. It precipitates out during the aging and can be considered the most important alloying element for the formation of precipitations in maraging steels. It was used as a precipitation-promoting element in the first maraging steels that were developed and is used today in complex alloying systems.

The greatest advantage is the rapid precipitation; titanium is thus much more active, for example, than Mo in C-type and T-type maraging steels in the early stages of precipitation. The enormous influence of the Ti content on the tensile strength 18% Ni and Co-containing maraging steels is shown in FIG. 1. In addition, small quantities of Ti are added to Ti-free maraging steels in order to form carbides. The objective is to bind to the carbon C so that no other precipitation elements can form carbides.

The influence of the Ti content is shown in FIG. 1.

Molybdenum (Mo):

With an increasing Mo content, an increase in the hardness after aging can be observed (FIG. 2) since Mo forms intermetallic compounds with Ni. The precipitation behavior of Mo is strongly influenced by other elements, especially by cobalt (Co), among others. The addition of Co decreases the solubility of Mo in the matrix and Mo is also forced to form precipitations. This leads to an increase in the hardness (FIG. 2). Furthermore, Mo also increases the solid solution strengthening (FIG. 2) and improves the corrosion resistance of high Cr-containing maraging steels.

The influence of the Mo content is shown in FIG. 2.

Chromium (Cr):

Chromium is added to improve the corrosion resistance of maraging steels. This yields steels that can be used, for example, as plastic mold steels, which are exposed to a chemical attack during the production of plastics. The addition of Cr to the alloy promotes the precipitation of the Laves phase. But higher Cr contents can lead to the formation of the σ phase, which has a negative effect on the mechanical properties. Furthermore, in long-term aging, spinodal segregation into Fe-rich and Cr-rich phases can occur, which reduces the notch impact strength.

Manganese (Mn):

In order to develop economical maraging steels, Mn was sometimes used to replace the more expensive Ni. Consistent with Ni, Mn forms a Mn martensite, but has less of an austenite-stabilizing effect and thus a significant quantity of δ ferrite is present in Fe—Mn alloys. This δ ferrite has a negative effect on the mechanical properties and on the corrosion resistance.

It is also already known that Mn forms intermetallic compounds with Fe and Ni.

Carbon (C):

Carbon is not an alloying element of a maraging steel. Because maraging steels cannot obtain their high strength from carbides, the carbon content is kept as low as possible during the production of the steel. For this reason, the carbon content of a maraging steel is in the range of 1/100%.

The corrosion resistance and the weldability deteriorate when carbon forms Cr-carbides in stainless maraging steels. In PH 13-8 Mo maraging steels, C forms carbides with Mo and Cr.

Copper (Cu):

Cu acts as a precipitation-promoting element in maraging steels; it does not, however, form a compound with other elements. At the beginning, it precipitates out with a cubic, body-centered structure in the Fe matrix. During the aging, it develops a 9R structure and in the end, it forms its cubic face-centered structure in equilibrium. The role of copper is to rapidly precipitate out and serve as a nucleation site for other precipitations.

Silicon (Si):

Silicon is usually considered an impurity element in steels. But in maraging steels, Si forms intermetallic phases and particularly in alloys that contain Ti, it forms the so-called Ni16Si7Ti6 G phase. The term “G phase” is used because the phase was discovered for the first time at grain boundaries; this is not the case, however, in maraging steels.

The good mechanical properties of maraging steels can be attributed to a two-stage heat treatment.

FIG. 3 shows an example of a time/temperature diagram of such a heat treatment, which consists of a solution annealing procedure followed by an air-cooling to room temperature and an aging process.

If after the solution annealing procedure, a quenching from the austenitic monophase field is carried out, then a soft, but powerfully distorted Ni-martensite is formed, which can be easily machined and cold-worked if need be. The subsequent aging is typically carried out in a temperature range of 400° C. to 600° C. During the aging process, three reactions occur:

-   -   (i) precipitation of intermetallic phases     -   (ii) recovery of martensite     -   (iii) formation of retransformed austenite

The precipitation of high-nm intermetallic phases is responsible for the immense increase in strength after the aging. Maraging steels have a series of essential advantages:

-   -   only a two-stage heat treatment is required     -   complex shapes can be easily machined in the un-aged state     -   subsequent hardening with minimal deformation

The phase diagram of Fe—Ni in equilibrium is shown in FIG. 4. It clearly shows that Ni decreases the conversion temperature from austenite to ferrite and that in the alloys that contain more than a few percent Ni, the structure in equilibrium at room temperature consists of austenite and ferrite.

However, in practice under real cooling conditions starting from the austenitic monophase, the material does not decompose into a composition of austenite and ferrite in equilibrium. Instead, the austenite, with further cooling, is transformed into a cubic martensite.

The aging of the martensitic structure is possible due to the influence of Ni in maraging steels, which leads to a hysteresis of the conversion temperatures of martensite and austenite during the heating and cooling (FIG. 5). With an increasing Ni content, the conversion temperature of heating and cooling decreases. In this connection, the difference between the conversion temperatures depends on the Ni content.

After the solution annealing, the material is transformed to a martensite if it is cooled to below the conversion temperature. Depending on the Ni content and the other alloying elements, a certain percentage of austenite can be retransformed at room temperature. If the microstructure is reheated again to below the α-γ conversion temperature, then the martensite decomposes into an equilibrium structure composed austenite and ferrite. The speed of this reconversion reaction depends on the temperature used. Fortunately in maraging steels, this conversion is slow enough that precipitations of the intermetallic phases from the over-saturated solution form before the reconversion reaction dominates.

If on the other hand, the alloy is heated to above the α-γ conversion temperature, then the martensite is retransformed due to annealing processes.

The alloy concept according to the invention is essentially based on a concept that is built on Ni, Al, Ti, and Si as hardening elements (FIG. 6 for sample steels, FIG. 7 for the range according to the invention).

In order to increase the hardness, strength, and toughness of the alloy, the focus of the alloy development was placed essentially on two points:

-   -   An increase in the hardness and strength values was achieved by         modifying the precipitation densities and types. The contents of         the precipitation-promoting elements Al and Ti were also         increased.     -   In order to increase the toughness, the percentage of         retransformed austenite was increased. It was possible to         achieve this by increasing the Ni content.

In sum, it is clear that with the alloy according to the invention, by maintaining the given ranges of temperature for the solution annealing and for the aging, a steel material is achieved that has a high level of hardness along with a surprisingly high retransformed austenite content. It is thus possible, along with the very high level of hardness, to achieve a high level of toughness so that with regard to the toughness, a minimum level of 25 J in the notched bar impact work is achieved.

According to the invention, the solution annealing treatment takes place at 900-1000° C. for 0.5-1.5 hours.

The aging takes place at 475° C. to 525° C. for two to six hours.

The invention will be described in greater detail based on the examples below.

A steel material with chemical compositions according to FIG. 6 is melted in the vacuum induction furnace and possibly remelted by means of ESR (electroslag remelting) or VAR (vacuum arc remelting) and correspondingly cast into sample bodies.

In the form of these sample bodies, the material is then characterized in different heat treatment states with regard to its structure, hardening/aging behavior, and mechanical properties.

The solution annealing was carried out at 1000° C. for 1 h and the aging was carried out 3 h at 525° C. The hardness was then determined using the Rockwell method.

FIGS. 8 and 9 show the characteristic values of a plurality of alloys with regard to hardness and toughness.

In this connection, in both of these figures, the alloys V21, V311, V321, and V322 according to Table in FIG. 6 correspond to the alloys according to the invention.

FIG. 8 shows that the corresponding alloys according to the invention lie in the upper range of hardness of all of the alloys and thus have absolutely sufficient hardness properties.

FIG. 9 shows the percentage of austenite as a function of the aging temperature. In this case, different percentages of retransformed austenite were produced, the retransformed austenite being responsible for the toughness of the material. It is clear that the alloys according to the invention are all very close to one another, particularly with an aging temperature of 525° C., and the percentages of austenite are absolutely sufficient for the high level of toughness.

If one compares this to the comparison alloy, it is clear that there are indeed alloys that have a high percentage of austenite, but if this is compared to FIG. 8, it is clear that they fall short in terms of hardness. Other alloys have a significantly lower percentage of austenite or even no austenite whatsoever and are thus very poor in toughness, even though they are better in hardness.

It is therefore clear that the invention enables a particularly successful combination of hardness and toughness. 

1. A method for producing an article out of a maraging steel, comprising the steps of: providing a maraging steel having the following composition in M % weight percent: C=0.01-0.05 Si=0.4-0.8 Mn=0.1-0.5 Cr=12.0-13.0 Ni=9.5-10.5 Mo=0.5-1.5 Ti=0.5-1.5 Al=0.5-1.5 Cu=0.0-0.05 residual iron and smelting-induced impurities; producing a form from the steel; and successively subjecting the form to a solution annealing and aging heat treatment to produce the article.
 2. The method according to claim 1, further comprising the steps of: melting the steel in a vacuum induction furnace; remelting the steel using VAR or ESR; and casting the steel into the form.
 3. The method according to claim 1, wherein the solution annealing is performed at 900-1000° C.
 4. The method according to claim 1, wherein the solution annealing is performed for 0.5-1.5 hours.
 5. The method according to claim 1, wherein the aging heat treatment is performed at 475° C.-525° C.
 6. The method according to claim 1, wherein the aging heat treatment is performed for two to six hours.
 7. The method according to claim 1, wherein after the solution annealing and aging heat treatment, the steel has a hardness of >50 HRC.
 8. The method according to claim 1, the aging heat treatment is performed at a temperature of 525° C., and the steel after aging has a retransformed austenite content of between 4 and 8 vol. %.
 9. An article comprising a maraging steel, wherein the steel has the following composition in M % weight percent: C=0.01-0.05 Si=0.4-0.8 Mn=0.1-0.5 Cr=12.0-13.0 Ni=9.5-10.5 Mo=0.5-1.5 Ti=0.5-1.5 Al=0.5-1.5 Cu=0.0-0.05 residual iron and smelting-induced impurities; wherein the article is produced by forming, solution annealing and aging the steel under heat treatment.
 10. The article of claim 9, characterized in that it has a hardness of >50 HRC and a percentage of retransformed austenite of 4 to 8 vol. %.
 11. The article of claim 10, wherein the steel comprises: C=0.01-0.03.
 12. The article of claim 10, wherein the steel comprises: Si=0.4-0.8.
 13. The article of claim 10, wherein the steel comprises: Mn=0.1-0.3.
 14. The article of claim 10, wherein the steel comprises: Cr=12.2-12.5.
 15. The article of claim 10, wherein the steel comprises: Ni=9.8-10.2.
 16. The article of claim 10, wherein the steel comprises: Mo=0.8-1.2.
 17. The article of claim 10, wherein the steel comprises: Ti=0.8-1.2.
 18. The article of claim 10, wherein the steel comprises: Al=0.5-1.1.
 19. The article of claim 10, wherein the steel comprises: Cu=0.02-0.04.
 20. A method for producing an article out of a maraging steel, comprising the steps of: providing a maraging steel having the following composition in weight percent: C=0.01-0.05 Si=0.4-0.8 Mn=0.1-0.5 Cr=12.0-13.0 Ni=9.5-10.5 Mo=0.5-1.5 Ti=0.5-1.5 Al=0.5-1.5 Cu=0.0-0.05 residual iron and smelting-induced impurities; producing a form from the steel; and successively subjecting the form to a solution annealing at 900° C. for 0.5-1.5 hours and aging heat treatment at 475-525° C. for two to six hours to produce the article; wherein the article has a hardness of >50 HRC and a percentage of retransformed austenite of 4 to 8 vol. %. 